More is not always better: Peat moss mixtures slightly enhance peatland stability

Description

Notes

Methods

Early September 2021, we collected 40 cores (Æ 22.5 cm, 15 cm depth) of acrotelm peatmoss communities from a Sphagnum­-dominated peatland in the Store Mosse National Park, Sweden (57°17'54 N, 14°00'39 E). Our sampling strategy was set up to create a single-density replacement series (Jolliffe, 2000) with two naturally co-occurring Sphagnum mosses: S. cuspidatum Ehrh. ex Hoffm and S. medium Limpr. These species grow in close proximity to each other in the field and make up >70% (cover) of the field layer in the field site. Our sampling strategy resulted in cores with monocultures (100% cover) of S. cuspidatum or S. medium, and three mixtures of S. cuspidatum and S. medium – 75%/25%, 50%/50%, 25%/75% (n = 8). Final cover ratios may differ by ± 5%. Vascular plants were carefully removed

Following and acclimation period, the eight mesocosms for the five compositionally distinct Sphagnum communities were randomly assigned an experimental drought treatment: mild drought or deep drought (n = 4). The mild drought consisted of a water table drawdown of 5 cm followed by a seven-day drought period in which no precipitation was added. The deep water table drawdown was similar except that the initial seven-day mild drought was followed by a water table drawdown of 20 cm ­– hence a full drainage ­– and an additional two-week drought. Every drawdown event was preceded by mimicking a heatwave, which under natural conditions precedes a drop in water table­. The heatwave was realized by increasing the climate room temperature to c. 35°C for 10 hrs. using twelve infrared lamps (Philips IR 250 RH IR2 230-250V 250W) that were installed 70 cm above the surface of the mesocosms. Both drought treatments were trailed by a rewetting period (= recovery period) which lasted eight weeks.

Carbon dioxide (CO₂) concentrations were measured throughout the experiment using an acrylic, cylindrical, transparent chamber (ID 29 cm, height 30 cm) that could be placed air-tight on the mesocosms using a rubber inner sealing. The chamber was equipped with an internal fan. On measurements days – two times per week in the acclimation period; four times per week in the drought period, two times per week in the recovery period – the transparent chamber was connected in a closed loop to a laser-based gas analyser that uses Optical Feedback – Cavity-Enhanced Absorption Spectroscopy to measure gases in air (LI-7810 CH₄/CO₂/H₂O Trace Gas Analyzer, LI-COR Biosciences). Gas measurements were performed with a frequency of 1 hz over a 120 s interval. The chamber was vented between measurements.

Measurements with the transparent chamber represent Net Ecosystem Exchange (NEE). CO₂ fluxes were calculated using the R package FluxCalR (Zhao, 2019), which makes use of the change in gas concentration in the chamber over time. Due to instability in the gas flux measurements, we used a dead band of 30 s for the NEE calculations. The ecological sign convention was used for the NEE data; hence, positive flux values indicate CO₂uptake, while negative flux values indicate CO₂ loss to the atmosphere.

We calculated ecosystems stability measures for all individual mesocosms based on the framework described by Isbell et al., 2015 (Nature, 526, 574–577. https://doi.org/10.1038/nature15374).